A look at the type of bomb that may have been used in the terror attacks at the Boston Marathon

Pressure cookers – possibly activated remotely by a cellphone – are believed to have been used to make the crude bombs that sent deadly shrapnel hurling into a crowd of onlookers and competitors at Monday’s Boston Marathon, experts told Fox News.

Doctors treating some of the 176 injured victims believe the explosives were packed with deadly shrapnel, including pellets, nails and sharp metallic objects – with some patients having “40 or more” such fragments embedded in their bodies.

“Many of them have severe wounds mostly in the lower part of their body – wounds related to the blast effect of the bomb, as well as small metallic fragments that entered their bodies – pellets, shrapnel, nails – that these bombs had,” George Velmahos, chief of trauma surgery at Massachusetts General Hospital, said Tuesday.

“I wouldn't exclude completely the possibility that some of these fragments are environmental, but my opinion is that most of them were in the bomb,” Velmahos said.

“They are numerous,” he added. “There are people who have 10, 20, 30 or 40 of them in their body or more.”

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FBI special agent in charge Richard DesLauriers says pieces of black nylon and fragments of ball bearings and nails were found, and authorities believe the bombs were placed in a dark-colored backpack or bag.

Sources close to the investigation say shredded pieces of pressure cookers were found at the blast sites, indicating the bombs were made from kitchen devices.

A source close to the investigation told Fox News that the style of the device used closely resembles that commonly seen in Afghanistan and Pakistan. Another source confirmed that "pressure cookers were used in both devices."

The source, who spoke on condition of anonymity, said a pressure cooker was attached to a wooden board in at least one of the blasts. The pressure cooker acted as the timer, the source said, and attached to the board was a bottle filled with nails, ball bearings and BB’s. That device was placed inside a black nylon backpack and then dumped in a garbage can, according to the source.

A third source said that the FBI is scanning cellphone tower records for the "moment of detonation." If the moment of detonation is positively identified, the corresponding cellphone number can be traced.

Pressure cookers have been used to make bombs in other high-profile cases, including the May 2010 thwarted attack in Times Square. The Department of Homeland Security warned of their potential use in terrorist attacks as early as 2004.

"I don't think the use of pressure cookers points to any particular group or individual who may be responsible," Bill Daly, a former FBI investigator, told FoxNews.com.

Jeff Lanza, another retired FBI agent, said the Boston case "bears striking similarities" to the bombings at the 1996 summer Olympic Games in Atlanta, in which Eric Robert Rudolph planted three pipe bombs with “lots of shrapnel,” killing two and injuring 111 people.

Lanza also said the smoke color seen at an explosion – which in the Boston case was white – will help indicate the type of bomb used in the blasts.